Air core motor-generators utilizing a double rotating construction have been developed. These electrical machines eliminate magnetic induced power losses in a steel stator and provide for high efficiency through elimination of the steel stator. A circumferentially varying magnetic flux thereby does not pass through a stationary steel structure and hence no hysteresis or eddy current losses in stationary steel result. An additional benefit of air core motor-generators is that they have much lower inductance than conventional slot wound machines. This low inductance can allow for easier high-speed operation as well as faster current rise times and potentially higher power output when operating a generator.
Double rotating air core motor-generators have mostly been used in small and high speed motor applications such as for use in cameras, disc drives and similar products. New versions of these electrical machines are currently being developed for use in larger products such as for industrial and commercial applications. Unfortunately, these new motor-generators are encountering several deficiencies including unpredictable armature failures, less than optimal windings densities resulting in reduced efficiency potential and windings that are very difficult to construct.